The present invention relates to anti-corrosion protective coatings for metal surfaces, e.g. metal pipes that are destined for inground implantation, and more particularly, to certain improvements over the invention described and claimed in U.S. Pat. No. 4,472,231 issued Sept. 18, 1984 to Robert F. Jenkins and assigned to The Kendall Company, the assignee of the instant application.
It has previously been the practice to provide anti-corrosion protective pipe coatings by supplying, in roll form, preformed polyolefin tapes having one surface that is coated with a butyl-based adhesive comprising a mixture of both virgin butyl rubber and reclaimed butyl rubber.
U.S. Pat. No. 4,268,334 issued to George M. Harris and Samuel J. Thomas discloses a method for increasing the strength of the adhesive bond in a layer comprising a blend of virgin butyl rubber and reclaimed butyl rubber comprising the step of incorporating in the butyl adhesive layer specified amounts of a p-dinitrosobenzene or p-quinone dioxime crosslinking agent and an activator such as lead oxide. As is stated in Col. 1, lines 26 et seq., the amount of crosslinking agent employed is in excess of a threshold value. However, it is stated at Col. 1, lines 24-26 that the amount of crosslinking agent has no appreciable effect upon bond strength of the tape after it has been applied to the pipe. Accordingly, no additional crosslinking of the rubber in the underlying primer or the primer-adhesive interface regions would occur subsequent to the application of the adhesive tape to the pipe itself.
The aforementioned U.S. Pat. No. 4,472,231 of Jenkins is directed primarily to providing an improved anti-corrosion protective coating for the surface of metal pipes that are designed for inground implantation, e.g. an improvement over procedures such as are disclosed in the aforementioned U.S. Pat. No. 4,268,334 of Harris et al.
As explained in the Jenkins patent, anti-corrosion protective coatings that are applied to inground pipeline structures are often subjected to rather severe long-term shearing forces derived from the surrounding soil. The magnitude of these shearing forces depends upon several factors, including amongst others: (a) the type of the soil, (b) the tectonic forces surrounding the implanted pipeline, (c) the size of the pipe, (d) the axial site emplacement and (e) the range of thermal expansion of the pipe as well as its contents.
In order to understand how each of the above factors affect the overall shear stress imparted to an inground pipeline coating, we first shall consider the forces acting upon implanted pipelines.
Frictional forces acting between the pipeline anti-corrosion protective coating and the surrounding soil are the primary source of shear stress. Frictional forces are here defined as the product of the frictional coefficient between the pipeline coating and the soil and the normal force acting around the pipe. As the coefficient of friction depends upon both the nature of the pipeline coating as well as the surrounding soil, it will be found to vary in different applications. Olefin polymer pipeline protective coatings, such as polyethylene, or the like, inherently exhibit lower coefficients of friction, as the protective tape outer surfaces are smooth and substantially non-adherent.
Other factors having importance in these considerations are the weight of the soil above the pipe, as well as the weight of the pipe, including its contents. In addition, since the normal force will vary depending on the axial position around the pipe diameter, the frictional force and hence the shearing force, will also be found to vary around the diameter of the pipe.
The result of long-term shear forces on a pipeline protective coating is referred to as "soil stress." Soil stress on anti-corrosion protective coatings generally results from the structural shear forces which cause the protective coating to creep along the pipeline peripheral surface.
Creep is, in essence, a long term visco-elastic, or "cold-flow" phenomenon, common to all polymeric substances. The amount of creep, however, will depend upon the physical properties of a coating. Since the physical properties (i.e. modulus) of a coating, will be temperature dependent, temperature becomes a decisive element in determining the amount of creep. At low temperatures, the propensity of the protective coating to creep will be substantially reduced, while at elevated temperatures, the likelihood of creep will be significantly increased, other factors remaining the same.
However, adhesive resistance to flow or creep, may be improved by introducing crosslinks between the component rubber chains.
When a rubber-based, or the like, adhesive system is crosslinked, (1) its resistance to creep is increased, (2) the overall dimensional stability is improved, and (3) it is more resistant to heat distortion. In addition, the above-listed crosslinking effects are generally intensified as the crosslink density is increased, and can therefore be controlled by adjusting the number of crosslinks in an adhesive coating. Crosslinking provides numerous anchoring points for the individual rubber chains, and these anchor points restrict excessive movement within the rubber of the adhesive, thereby resulting in limited creep or flow of the polyolefin tape coating.
As further stated in the Jenkins patent, a typical conventional pipewrap anti-corrosion protective system may comprise a primer that is applied to the pipeline outer surface, and an anti-corrosion protective adhesive tape overlaying the primer coating. The primer in the conventional pipewrap anti-corrosion system is typically a mixture of rubber and resins, which may be applied to the pipeline outer surface, by means of spraying, brushing, dipping or rugging. The adhesive tape is generally composed of a polyolefin backing material, such as polyethylene, or the like, with a rubber-based adhesive that is coated onto a surface of the polyolefin backing material. In the conventional pipewrap anti-corrosion protective system there is no crosslinking agent present in either the primer or the adhesive tape components.
According to the teachings of Jenkins, improved anti-corrosion protective coatings may be provided for metal pipes that are to be subjected to a high shear stress inground environment by what the patentee terms a two component interacting pipewrap anti-corrosion protection system consisting of: (1) a primer mixture comprising a blend of natural rubbers, resins and a crosslinking metal oxide activator coated with organo-titanate; and (2) a rubber-based adhesive carried on an olefin polymer backing to be helically wrapped over the primer layer, the adhesive layer comprising blend of virgin butyl rubber and reclaimed butyl rubber partially crosslinked with p-quinone dioxime crosslinking agent, a tackifier, and a residual amount of the p-quinone dioxime crosslinker.
The organo-titanate surface treated metal oxide, preferably lead dioxide, serves primarily as a crosslinking catalyst to increase both the speed and yield of the crosslinking reaction effected by the p-quinone dioxime crosslinking agent. As is explained at the top of Col. 3 of the Jenkins patent, if the lead dioxide is not surface-treated with the organo-titante coupling agent, the crosslinking reaction will take place at a considerably slower rate, thereby severely limiting the usefulness of the metal oxide as a crosslinking catalyst.
Initial partial crosslinking in the described two component interacting pipewrap system occurs only in the adhesive component layer. However, as the adhesive layer, containing residual, unreacted p-quinone dioxime crosslinker, is placed in contact with the primer-coated pipe outer surface, a further crosslinking reaction then occurs at the primer-adhesive interface, as well as throughout the primer layer and the adhesive layer. This additional crosslinking, which is aided in part by the elevated temperature of the operating pipeline and its contents, results in an improved ability of the system to resist tape creep caused by high shear forces.
As is stated in Col. 3, the crosslinking reaction results in increasing the cohesive strength, and consequently in shear resistance, of both the adhesive and primer layers. The primer-adhesive interface zone of crosslinking following application of the adhesive-coated tape to the primer-coated pipe surface results in an inter-crosslinked adhesive/primer system. An important feature is the crosslinking at the adhesive/primer interface which serves to increase markedly the adhesion of the helically wrapped tape to the primer-coated pipe, thereby reducing significantly the creep caused by high shear stress forces in situ. Further, the increase in the speed and extent of the crosslinking reaction rate results in the above-described improved anti-creep characteristics in the presence of soil shear forces.
It will be noted that the two-component system of Jenkins relies upon what the patentee describes in essence as a high speed additional crosslinking obtained by employing p-quinone dioxime as crosslinker and metal oxide, preferably lead dioxide, activator surface-treated with organo-titante. The increased speed obtained thereby was thought to be critical to the solution of the task of the invention.
While the patented system was entirely satisfactory in small-scale manufacture of an anti-corrosion pipewrap system, it nevertheless suffered from certain deficiencies making it impractical in the larger scale commercial manufacture of the system.
Specifically, it has been found that the operating conditions taught in U.S. Pat. No. 4,472,231 do not provide a procedure which is processable in a Banbury mixer in commercial production of the adhesive. Repeated attempts to implement the teachings of the '231 patent on production equipment immediately resulted in lumpy adhesive.
The present invention is directed to the solution to this problem, i.e. to adapt the teachings of '231 to a commercially feasible continuous production run to provide a pipewrap meeting the desired shear requirements, e.g. after 48 hours conditioning at 85.degree. C., the shear rate will not exceed 10-.sup.8 meters/second.
In order to solve this lumping problem, it was first necessary to ascertain the cause of the problem. Accordingly, the present invention is in part predicated upon the recognition and clear understanding of the cause and nature of the problem; and it is in part predicated upon the discovery of a means for obviating the problem.